Structures and operations of a conventional optoelectronic device are described below referring to FIGS. 17, 18 and 19.
FIG. 17 shows a cross section of an optical system and an information-recording medium of a conventional optoelectronic device. In FIG. 17, a broken line indicates how a light beam emitted from a semiconductor laser 1 and a light beam reflected at an information-recording medium 5 spread. In FIGS. 17 and 19, cross sections are not hatched for specifically indicating the light paths.
The light beam emitted from the semiconductor laser 1 as a light source enters a hologram optical element 2 having a diffraction grating region 8 as shown in FIG. 18. The 0th-order diffracted light is converged on the information-recording medium 5 by a collimator lens 3 and an objective lens 4, and reflected. The light beam reflected on the information-recording medium 5 is guided by the objective lens 4 and the collimator lens 3 to the diffraction grating region 8 of the hologram optical element 2. The reflected light is diffracted again there so that the ±first-order diffracted light enters photodetectors 6. The reflected light beam guided to the photodetector 6 is detected and calculated so as to detect a reproduction signal and various servo signals.
In FIG. 17, an alternate long and short dash line indicates an optical axis 10 of the light emitted from the semiconductor laser 1. FIG. 18 shows an effective light beam 9 of the emitted light passing through the diffraction grating region 8.
In the above-identified conventional optoelectronic device, various signals are detected by selecting among light beams emitted from the semiconductor laser 1, exclusively 0th-order diffracted light that passes through the hologram optical element 2.
However, a part of the emitted light entering the hologram optical element 2 is subject to ±first-order diffraction and mixed with the 0th-order diffracted light. That is, as shown in FIG. 19, a part of the ±first-order diffracted light is converged on the information-recording medium 5 and reflected to return to a region where the diffraction grating region 8 is not formed on the hologram optical element 2, and it passes through the hologram optical element 2 so as to enter the photodetectors 6. For clarification, FIG. 19 shows only +first-order diffracted light received by the photodetector 6, among +first-order light beams that are generated at the diffraction grating region 8.
In other words, since a light beam other than the normal signal light (the 0th-order diffracted light of a light beam emitted from the semiconductor laser 1) becomes stray light 20 to enter the photodetector 6, a signal/noise ratio (S/N ratio) deteriorates during reproduction of the information-recording medium 5, resulting in degradation in the reproduced signal and instability of the various servo controls.
In view of the above problems, the object of the present invention is to provide an optoelectronic device that obtains an excellent reproduction signal while allowing a stable servo control by providing a means for preventing deterioration of the S/N ratio caused by stray light.